Conventionally, image displays such as liquid crystal displays use polarizing plates comprising a laminate of a polarizer and a protective film such as a triacetyl cellulose film. The polarizer is produced by dyeing a polyvinyl alcohol-based film with a dichroic dye such as iodine and then stretching it in an aqueous boric acid solution. The hue of the polarizing plate significantly influences the due of liquid crystal displays.
Conventional polarizing plates, however, have low transmittance at wavelengths from 400 nm to 500 nm with respect to the transmission spectrum in the parallel Nicol configuration and thus show a yellow hue in the parallel Nicol configuration. On the other hand, high transmittance is performed at wavelengths from 400 nm to 500 nm with respect to the transmission spectrum in the crossed Nicol configuration and thus shows a blue hue in the crossed Nicol configuration. Thus, there has been a problem that conventional liquid crystal displays using such polarizing plates can become slightly yellow when white viewing and can become blue when black viewing.
As a method of producing neutral gray display when white viewing or black viewing, a method is disclosed including treating an iodine-dyed, uniaxially-stretched, polyvinyl alcohol-based film in an aqueous boric acid solution containing a specific amount of potassium iodide and controlling the treatment temperature in each step (for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-169024). According to the method as disclosed in the publication, however, the resulting hue is not sufficiently good in each of the parallel and crossed Nicol configurations.
There has also been a problem that iodine-type polarizers do not have sufficient durability and two pieces of such polarizers in the crossed Nicol configuration can cause polarization dropout at long wavelengths and can be discolored red when placed in a heated environment. About this problem, a polarizer containing an appropriate amount of zinc ions is proposed (for example, Japanese Patent Nos. 1308919 and 1606999 and JP-A No. 2002-35512). These patent literatures disclose that long wavelength polarization dropout can be prevented when polarizers are allowed to stand under high temperature conditions. According to these patent literatures, however, it is needed to make polarizers contain zinc ions, and thus there are problems about the zinc precipitate on the surface of polarizers, the complicated control of concentration, and skin irritation.